Evidence for interspecific competition between feral ass Equus
asinus and mountain sheep Ovis canadensis in a desert environment
Jason P. Marshal, Vernon C. Bleich & Nancy G. Andrew
Marshal, J.P., Bleich, V.C. & Andrew, N.G. 2008: Evidence for interspecific competition between feral ass Equus asinus and mountain sheep
Ovis canadensis in a desert environment. - Wildl. Biol. 14: 228-236.
Different ungulate species that co-occur over evolutionary time have
been hypothesized to develop mechanisms to limit the degree to which
they directly compete for the same limited resources (i.e. resource partitioning). In situations where an exotic ungulate has been recently introduced to a system, resource partitioning has not likely developed;
this appears to be the situation between introduced feral ass Equus asinus and indigenous mountain sheep Ovis canadensis in the Sonoran
Desert of southeastern California, USA. We analysed data from aerial
surveys conducted during 1993-2003 to look for evidence of an effect of
feral ass abundance on mountain sheep demography. After controlling
for the influence of forage availability via rainfall, we found no evidence
of relationship between an index of feral ass abundance and indices of
reproduction or recruitment in mountain sheep (Pi0.360). However,
we found strong evidence for an effect on indices of sheep abundance
and population rate of change. There was an interactive effect of rainfall and feral ass, such that a negative relationship between abundances
of mountain sheep and feral ass was strongest during the driest years
(P=0.014). There was a negative relationship between rate of change
for sheep populations and feral ass abundance (P=0.016), which was
not affected by rainfall. These results indicated a competitive effect of
feral ass on mountain sheep populations, but the mechanism of competition remains unclear. Further research should use manipulations in
feral ass abundance to clarify interactions between these species.
Key words: California, Equus asinus, exotic large herbivore, interspecific
competition, invasive species, Ovis canadensis, Sonoran Desert
Jason P. Marshal*, School of Natural Resources, 325 Biological Sciences
East, University of Arizona, Tucson, AZ 85721-0043, USA - e-mail: jason.
[email protected]
Vernon C. Bleich, Sierra Nevada Bighorn Sheep Recovery Program, California Department of Fish and Game, 407 West Line Street, Bishop, CA
93514, USA - e-mail: [email protected]
Nancy G. Andrew, California Department of Fish and Game, 78-078 Country Club Drive, Bermuda Dunes, CA 92201, USA - e-mail: nandrew@
dfg.ca.gov
*Present address: Centre for African Ecology, School of Animal, Plant and
Environmental Sciences, University of the Witwatersrand, Wits 2050, South
Africa
228
Ó WILDLIFE BIOLOGY 14:2 (2008)
Corresponding author: Jason P. Marshal
Received 29 October 2006, accepted 30 March 2007
Associate Editor: David Cumming
Mountain sheep Ovis canadensis in the deserts of
southeastern California have exhibited decreasing
populations in recent years (Epps et al. 2003). As
with populations of wild large herbivores in many
arid regions, loss of sheep habitat caused by human
activity (Bleich et al. 1996), changes in forage conditions in response to climate (Bleich et al. 1990), and
competition from exotic large herbivores (Bleich
2005) have all contributed to an ongoing conservation problem. Mountain sheep occur widely over
much of southern California (Bleich et al. 1990).
They exhibit specialized habitat use, occupying
mountainous regions having escape terrain provided by steep, rugged slopes (Krausman et al. 1989,
Bleich et al. 1997). They avoid areas of low visibility
containing dense vegetation that can hide ambush
predators, while requiring adequate forage in the
form of forbs and browse (Valdez & Krausman
1999). This creates a naturally fragmented sheep
distribution where persistence of populations requires both favourable patches of habitat and corridors between patches and mountain ranges across
the larger landscape (Bleich et al. 1990, 1996, Singer
et al. 2000). This specialized habitat use has made
mountain sheep populations particularly vulnerable to anthropogenic disturbances, and competition with exotic feral ass Equus asinus is suspected to
exacerbate those influences (Bleich & Andrew 2000,
Bleich 2005).
Interspecific competition is believed to play an
important role in shaping animal communities,
including those of ungulates (Sinclair & NortonGriffiths 1982, Arsenault & Owen-Smith 2002).
Where different species of ungulates have been
sympatric through evolutionary time, mechanisms
are hypothesized to have evolved that allow for
coexistence between extant species (Putman 1996).
In the systems where habitat partitioning is not
facilitated by behavioural influences (i.e. interference competition), other mechanisms allow maintenance of sympatry. Through resource partitioning, the potential for exploitative competition is
lessened: different ungulate species maintain sympatry by specializing on different forage species or
different parts of the same forage species in the same
environment.
Ó WILDLIFE BIOLOGY 14:2 (2008)
The type of resources used by a species, particularly forage resources, has been related to body
size (Bell 1970, Jarman 1974). Among ruminants,
species-specific body size has lead to selection of
different foraging strategies, from highly selective
browsers with small body sizes and delicate mouths
that select high-quality browse and forbs to largebodied grazers that consume low-quality grasses
(Hofmann 1989). Based on these patterns, one
would expect two ungulate species of similar size
in the same community to show similar foraging
strategies with a greater potential for competition.
When a novel ungulate species is introduced into
an existing natural community, interspecific competition might become an issue for one or more
native species because selection for coexistence via
resource partitioning has not had the opportunity to
occur (Douglas & Leslie 1996). Introduced exotic
species can exhibit foraging strategies or body forms
that overlap those of the native ungulates. Thus,
depending on densities of exotic and native species
and, hence, resultant pressure on resources used in
common, potential for competition and exclusion of
one of the species is enhanced. This situation has
been hypothesized to exist between mountain sheep
and introduced feral ass in the arid southwestern
United States (Douglas & Leslie 1996).
Feral equids have increased throughout the
western United States since they were afforded
federal protection in 1971; that protection likely has
resulted in detrimental effects on native herbivores
with which those feral equids are sympatric (Bleich
2005). Indeed, concerns about impacts to arid systems by non-native ungulates have grown among
ecologists. Mountain sheep and feral ass are widely
recognized to use similar habitat (Dunn & Douglas
1982), and many plant species are common to the
diets of both (Ginnett & Douglas 1982). For example, in Death Valley National Monument, California, both species concentrate habitat use in
sage-brush (60-70%) and shade-scale (30-40%)
vegetation associations (Dunn 1984), and they exhibit common use of 67% of the forage species that
occur in the collective diets of both species (Ginnett
1982). Based on overlaps in distributions and diets,
feral ass might exclude mountain sheep from their
229
preferred habitats (Douglas & Leslie 1996). Nonetheless, one cannot conclude that overlap in diet
and range indicates competition between these species. Interspecific competition can occur only if
common use of limited resources affects the demography of one or both species (Wiens 1989, Putman
1996).
Based on differences in body sizes and foraging
strategies, one might not expect substantial overlap in resource use between mountain sheep and
feral ass. Adult body size of feral ass (147-158 kg
at Death Valley, California) is considerably larger
than that of mountain sheep (34-68 kg at Lake
Mead, Nevada; Douglas & Leslie 1996). As a result,
resource partitioning based on body size alone
might occur despite the recent introduction of feral
ass to North America. Nonetheless, both species fall
into the body size category defined by Jarman (1974)
as having intermediate selectivity associated with
intermediate body mass, and differing foraging
strategies (generalist vs specialist) and digestive
morphology (caecal vs ruminant) are apt to result in
diets of feral ass that overlap with those of mountain
sheep. That two species have very different body
sizes and digestive morphologies, however, does not
exclude them from the possibility of interspecific
competition (Belovsky 1984).
We used demographic data for sympatric mountain sheep and feral ass occupying the Sonoran
Desert to evaluate the hypothesis that feral ass were
having demographic impacts on mountain sheep
because of interspecific competition. We predicted
that there would be a negative relationship between
abundance of feral ass and reproduction, recruitment, population rate of change, and population
size of mountain sheep. Our analysis provided evidence for a competitive effect of feral ass on this
population of mountain sheep; there were negative
relationships between abundance of feral ass and
abundance and rate of change of mountain sheep.
Material and methods
Study area
Our study area was located in the East Chocolate
Mountains, Imperial County, California, USA,
where we investigated the demography of mountain
sheep during 1993-2003. This is a highly arid region
of the Sonoran Desert, located near the juncture of
the Colorado River and the international border
with Mexico (Fig. 1; Andrew et al. 1999). Average
230
Figure 1. Location of our study area in the East Chocolate
Mountains, California, USA.
rainfall was approximately 70 mm per year, and
summer high temperatures were regularly >45xC
(Imperial Irrigation District, Imperial, California,
unpubl. data). The size of our study area was approximately 1,400 km2.
The East Chocolate Mountains lie within the
Lower Colorado River subdivision of the Sonoran
Desert, and support vegetation common to that
region (Andrew 1994, Turner 1994). Mountainous
regions contained creosote bush Larrea tridentata,
brittle-bush Encelia farinosa, burro-weed Ambrosia
dumosa, and ocotillo Fouquieria splendens. In addition to these, riparian species, including salt cedar
Tamarix spp., cattails Typha domingensis, and
arrowweed Pluchea sericea, were common near the
Colorado River.
Within the study area, >90% of plant biomass
away from the river occurred in xeroriparian associations along dry desert washes (Marshal et al. 2005).
Common species in these associations were desert
ironwood Olneya tesota, palo verde Parkinsonia
florida, mesquite Prosopis glandulosa, and catclaw
Acacia greggii (Andrew et al. 1999). Other large- and
medium-sized herbivores in the area included mule
deer Odocoileus hemionus, black-tailed jackrabbit
Lepus californicus, desert cottontail Sylvilagus audubonii, and desert tortoise Gopherus agassizii. Removals of some feral ass occurred periodically
during our study, either by the United States Bureau
of Land Management (BLM) personnel or by illegal
Ó WILDLIFE BIOLOGY 14:2 (2008)
activity. Variation in observed feral ass abundance
might have been a result of either of those activities.
Data collection
We used readily identifiable ground features to
divide the study area into seven survey polygons
(Norton-Griffiths 1978). The total area was 295
km2, with polygons ranging in size within 22-56 km2
(x̄=42 km2). We completely surveyed each polygon
by flying contour lines separated by approximately
150 m elevation, at an average rate of 2.5 minutes/
km2 (Bleich et al. 1997). Annual survey flights
(x̄=9.0 hours; range: 8.0-10.2 hours) occurred over
two consecutive days in late September or early
October. Three observers and an experienced pilot
conducted the surveys using a Bell 206 B-III Jet
Ranger helicopter. The pilot and the senior observer
were the same for all the surveys. The other two
observers varied from year to year, but all were
experienced with aerial surveys, with the terrain
where the surveys occurred, and with the species
being counted. All observations of mountain sheep
and feral ass encountered during the surveys were
recorded. Individual mountain sheep were classified
(Geist 1968) as adult males (i.e. horn classes II-IV),
yearling males (i.e. horn class I), adult females,
yearling females or young-of-the-year.
Analysis
From the data collected during the aerial surveys,
we calculated indices of abundance, population rate
of change, reproduction and recruitment for use in
our analyses. For abundance, we calculated mountain sheep observed per hour (sheep/hour) and feral
ass observed per hour (ass/hour) from the total
number of each species observed during each annual
survey divided by the amount of time flown for that
survey. We calculated an index of population exponential rate of change (r) by taking the natural
logarithm of sheep/hour, and estimating successive
differences between each year. Because data were
unavailable for 2000, r for 2001 was the average rate
of change over two years. We used two indices of
reproduction: the number of young-of-the-year
(both sexes) observed per 100 adult females, and
the proportionofyoung-of-the-year (bothsexes)out
of all observed sheep. We used two indices of juvenile
recruitment: the number of yearlings (both sexes)
observed per 100 adult females, and the proportion
of yearlings (both sexes) out of all observed sheep.
With these indices, we investigated relationships
between mountain sheep demography and feral
ass abundance using linear regression. We evaluated
six models where the explanatory variables for all
models were rainfall over the year preceding the
survey (an index to forage availability), ass/hour on
surveys, and the interaction of these two variables.
We checked for collinearity between ass/hour and
rainfall, and found no evidence of a correlation
(Pearson’s R=0.24, two-sided P=0.50). Use of
rainfall in this manner did not account for the forage
that might have been consumed by feral ass. Thus,
including both factors in our analysis was necessary
to investigate overall forage effects, and then those
effects specific to feral ass. Use of rainfall as an index
to forage availability was supported by past research
in our study area (Marshal et al. 2005), which demonstrated an association between amount of precipitation and forage biomass (loge(forage biomass
(g/m2))=2.269+0.017 (previous six months’ rainfall (mm))-0.017 (temperature (xC)); R2=0.44, P<
0.001). We did not account for the influence of other
Table 1. Mountain sheep survey results from the East Chocolate Mountains, California, USA, during 1993-2003. The exponential
rate of change (r) was calculated as loge (Sheep/hour (at t) / Sheep/hour (at t-1)). No data were available for 2000, so the r-value
for 2001 is the average annual exponential rate of change over two years.
Proportion of
Year
Sheep/hour
r
Lambs/
100 ewes
Yearlings/
100 ewes
Lambs
Yearlings
Precipitation
(mm)
Feral ass/hour
1993
1994
1995
1996
1997
1998
1999
2001
2002
2003
11.1
5.8
6.1
3.2
4.4
5.9
3.6
4.6
3.9
5.1
-0.65
0.05
-0.64
0.32
0.29
-0.51
0.13
-0.18
0.29
43.5
11.4
5.9
54.5
20.0
77.3
41.2
55.6
37.5
56.3
13.0
0.0
2.9
9.1
26.7
4.5
35.3
5.6
0.0
12.5
0.19
0.05
0.03
0.19
0.08
0.33
0.21
0.24
0.19
0.22
0.06
0.00
0.02
0.03
0.11
0.02
0.18
0.02
0.00
0.05
160
72
80
21
17
128
73
57
11
72
7.4
7.2
4.7
7.5
4.4
3.6
5.0
2.6
3.0
3.5
Ó WILDLIFE BIOLOGY 14:2 (2008)
-----------------------------------------
231
Table 2. Relationships between bighorn sheep demographic measures and the influence of rainfall and abundance of feral ass in
the East Chocolate Mountains, California, USA, during 1993-2003. For calculation of the exponential rate of change (r), see the
legend of Table 1.
Response
Explanatory variable
Sheep/hour
Precipitation
Feral ass/hour
Precipitationrferal ass/hour
Feral ass/hour
Coefficient
SE
t
P
df
R2
-0.0218
-0.4901
0.0106
-0.1805
0.0183
0.2596
0.0031
0.0571
-1.191
-1.888
3.404
-3.159
0.279
0.108
0.014
0.016
6
0.91
7
0.59
herbivores in the system, choosing to treat their
influences as background noise that would occur
regardless of the influence of feral ass. Overlap in
habitat use between mountain sheep and the only
other large herbivore (mule deer) was small, with
sheep occurring in the more rugged terrain and
higher elevations of mountainous areas (Andrew et
al. 1999) and deer occurring in flatter areas near the
larger washes (Marshal et al. 2006). Further, mule
deer were not commonly observed during surveys;
thus, influence on interactions between mountain
sheep and feral ass likely were minimal. We did not
address in the models the specific influence of feral
ass removals, assuming that their effects on feral ass
abundance would be reflected in the index ass/hour.
The response variables for each of the six models
were 1) sheep/hour, 2) r, 3) young/100 females, 4)
log-odds transformed proportion of young, 5)
yearlings/100 females, and 6) log-odds transformed
proportion of yearlings. We used log-odds transformations to meet assumptions of linearity and
homogeneity of variances required for proportion
data (Ramsey & Schafer 2002).
Figure 2. Relationships between feral ass observed per hour and
mountain sheep observed per hour, based on surveys conducted
in the East Chocolate Mountains, California, USA, during19932003. Lines indicate predicted effects of feral ass on abundance of
mountain sheep at different levels of annual rainfall (in mm).
232
Results
The aerial surveys performed during 1993-1999 and
2001-2003, yielded 10 years of demographic information (Table 1). Sheep/hour ranged from 11.1
in 1993 to 3.2 in 1996, but generally fluctuated
between approximately four and six sheep/hour
from 1997 to 2003. Similarly, ass/hour fluctuated
from year to year, showing a decreasing trend from
7.5 in 1996 to 3.0 in 2002.
We did not detect an effect of forage availability or
abundance of feral ass on our indices of reproduction in mountain sheep. There was no evidence of a
relationship between rainfall or feral ass abundance
and young/100 females (F3,6=0.90, P=0.493).
Similarly, we detected no relationship between rainfall or feral ass abundance and log-odds (proportion
of young) (F3,6=0.43, P=0.741). Moreover, we did
not detect an influence of either forage availability or
abundance of feral ass on recruitment. Indeed, there
was no evidence for an effect of rainfall or feral ass
abundance on either yearlings/100 females (F3,6=
0.02, P=0.997) or log-odds (proportion of yearlings) (F3,6=0.14, P=0.930).
Figure 3. Relationship between feral ass observed per hour and
rate of change in the mountain sheep population, based on aerial
surveys conducted in the East Chocolate Mountains, California,
USA, during 1993-2003.
Ó WILDLIFE BIOLOGY 14:2 (2008)
Although reproduction and juvenile recruitment
did not appear to be influenced by either forage
availability (as indexed by rainfall) or abundance of
feral ass, there was strong evidence that those factors
influenced abundance and r of the mountain sheep
population (Table 2). There was an interactive effect
of rainfall and feral ass abundance on mountain
sheep abundance, such that there was a negative
association between ass/hour and sheep/hour when
rainfall was low, but the association became positive
with increasing rainfall (Fig. 2). Rate of change in
sheep was negatively related to ass/hour (Fig. 3),
such that the rate decreased by 0.18 (SE=0.06) for
each increment of one ass/hour.
Discussion
Our results for abundance and r of mountain sheep
are consistent with the hypothesis that feral ass have
a competitive impact on the mountain sheep population in the East Chocolate Mountains. Such
competition likely is greatest during years of low
rainfall (see Fig. 2), but additional survey data will
be necessary to strengthen this conclusion. Under
low-rainfall conditions, resource use by feral ass
would exacerbate the effects of competition on
limited forage and, consequently, on mountain
sheep. During years of high rainfall, both species
appeared to benefit from greater forage, which
would have substantially reduced competitive effects between feral ass and mountain sheep; consequently, the association between abundances of
mountain sheep and feral ass was positive during wet
years. A competitive effect of feral ass on mountain
sheep was also demonstrated in the rate-of-change
data (see Table 2), where there was strong evidence
for a negative relationship between abundance of
feral ass and change in abundance of mountain
sheep, even in the absence of an influence of forage
availability.
Further research is necessary to understand the
mechanism by which competition between feral ass
and mountain sheep occurs. If exploitative competition were the mechanism, we would have expected
relationships between abundance of feral ass and
reproduction or recruitment of mountain sheep, but
such relationships were not evident. Our data set
might have been too small to detect these influences,
a possibility supported by the failure to detect relationships involving forage availability. That there
was no evidence for an association between r and
Ó WILDLIFE BIOLOGY 14:2 (2008)
rainfall is counter-intuitive, considering the importance of rainfall in driving the population dynamics
of arid-land wildlife species (Wehausen et al. 1987,
Caughley 1987, Marshal et al. 2002).
It also is possible that the mechanism for competition between feral ass and mountain sheep was
behavioural; that is, mountain sheep were averse to
the presence of feral ass, but feral ass did not consume available sheep forage. For example, mountain sheep have been known to avoid drinking from
water sources being used by feral ass; some male
sheep waited hours for feral ass to leave a water
source before approaching to drink, and female
sheep generally would not drink at a source if >3
feral asses were present (Dunn & Douglas 1982).
These examples involve interactions at water
sources, but they suggest a larger tendency for
mountain sheep to avoid areas containing large
numbers of feral ass. As a result, behavioural
avoidance could have contributed to the negative
relationships between sheep and ass abundance that
we observed. During years of below-average rainfall, feral ass might concentrate near a smaller
number of wildlife water developments and force
sheep away from those areas, such that where
large numbers of feral ass were observed during
surveys, small numbers of mountain sheep also were
observed. Whether mountain sheep abandoned localized areas (e.g. in the vicinity of water sources) is
uncertain; nevertheless, the existence of a clearly
negative relationship between indices of abundance
of the two species suggests that some interaction
existed.
The traditional view is that if competition occurs
between feral ass and mountain sheep, it happens via
overlap in habitat and, subsequently, diet (Douglas
& Leslie 1996). Dunn (1984) reported substantial
similarities in all seasons in the use of vegetation
associations in the Cottonwood Mountains of
Death Valley National Monument, California. Further, Ginnett (1982) reported that of 55 forage taxa
used collectively by both species, 67% were used in
common. The degree of similarity between diets of
feral ass and mountain sheep varied between 20 and
61% in the Grand Canyon, Arizona (Walters &
Hanson 1978), and diets of feral ass and mountain
sheep demonstrated 40% similarity in spring and
50% during summer in western Arizona (Seegmiller
& Ohmart 1981, Ginnett 1982).
Based on diet similarity, many investigators have
recognized that the potential for competition could
exist between feral ass and mountain sheep (Douglas
233
& Leslie 1996). Nevertheless, simple niche overlap is
not sufficient to conclude the existence of competition, and Welles (1962) questioned the notion that
feral ass directly impact mountain sheep populations. Resources must be limiting to at least one
of the species, with resultant demographic consequences for at least one of the species. Our data
suggest competitive influences on population abundance and rate of change, but not on reproduction
or recruitment.
Deserts are resource-limited environments, where
population dynamics of large herbivores are driven
primarily by fluctuations in precipitation and resultant forage availability (Noy-Meir 1973, Caughley 1987, Marshal et al. 2002). Except during brief
periods at the end of a drought, when populations
are at low densities and forage is growing rapidly,
ungulate populations are likely to be resourcelimited (sheep/hour in our study area had a positive
correlation with rainfall alone (Pearson’s R=0.84,
two-sided P=0.003), consistent with this pattern).
In such an environment, feral ass might have an
advantage over mountain sheep because of their
generalist foraging strategy and monogastric digestive system (Janis 1976). This combination of traits
could make feral ass better able to persist on forage
of lower nutritional quality (i.e. with higher fibre
content)during timesofscarcitybyincreasingintake
rate and having a shorter digestive retention time.
Mountain sheep, as ruminants, would be less able to
follow this strategy: a minimum nutritional quality
of forage is required to provide nutrients at a rate
sufficient to sustain those herbivores, plus faster
through-put of poor-quality forage would allow less
time for fermentation and subsequent nutrient
absorption (Demment & Van Soest 1985). Under
conditions of sympatry and when forage production
islow,feralassmightbemorelikelytomeetminimum
forage requirements than mountain sheep (Douglas
& Leslie 1996).
Evidence available to us suggests that competition between feral ass and mountain sheep is most
apt to occur when resources are limited in the
environment (see Fig. 2). Whether the mechanism
for competition is behavioural or food nicherelated, this finding is consistent with other studies
that have reported variation in diet overlap, and
where the greatest degree of overlap occurs during
the most limiting environmental conditions. For
example, guanaco Lama guanicoe and domestic
sheep Ovis aries in Patagonia, Argentina, demonstrated a greater degree of diet overlap in summer
234
than in spring, when forage resources were less
scarce (Baldi et al. 2004). Similar results have been
reported for marsupial herbivores and domestic
livestock occupying arid ecosystems in Australia
(Dawson & Ellis 1994), and between pronghorn
Antilocapra americana and domestic sheep in North
America during winter, when forage availability was
most limited (Schwartz & Ellis 1981).
Manipulative experiments could be used to test
relationships between competing ungulate species
in the region containing the East Chocolate Mountains; however, such experiments are rare. Hobbs
et al. (1996a,b) manipulated density of wapiti Cervus elaphus on grazing pastures during winter to estimate the effect of forage removal by wapiti on
pasture plant biomass and, subsequently, on cattle
Bos taurus growth and reproduction the following
spring. Those authors reported a substantial influence of wapiti density on pasture biomass. Furthermore, they reported that forage consumption by
wapiti beyond a threshold level caused a decrease in
cattle production (Hobbs et al. 1996b). Stewart
et al. (2002) manipulated cattle density on an
enclosed experimental range to study niche overlap
and resource partitioning among cattle, mule deer
and wapiti. In their system, presence of cattle
influenced how the two native ungulates used space
and habitat, with mule deer and wapiti avoiding
areas used by cattle. Stewart et al. (2002) concluded
that competition between cattle, deer and wapiti
likely resulted in spatial displacement of native cervids.
The region encompassing our study area contains
several feral ass management zones, where BLM
personnel periodically remove feral ass in an
attempt to limit impacts of feral ass on the desert
ecosystem. In a framework of adaptive management, such removals provide an opportunity to
more thoroughly test for an influence of feral ass
abundance on the demographics of mountain sheep.
Any such experiment would necessarily occur over a
long period of time and, ideally, would involve a
treatment area where removals occur and a control
area where density of feral ass remain unmanipulated. Such a design would be useful for separating
the effects of feral ass abundance from that of
fluctuating forage availability. Over time, comparisons between physiological condition, survival,
reproduction, juvenile recruitment, population
rate of change, or population abundance among
mountain sheep in treatment and control areas
would demonstrate whether there is an effect of
Ó WILDLIFE BIOLOGY 14:2 (2008)
abundance on feral ass. If feral ass do have a
competitive effect on mountain sheep, one or more
of those parameters should be higher in the treatment area, indicating the presence of interspecific
competition and more clearly demonstrating the
mechanism for that competition.
Acknowledgements - we are grateful to P. Duncan and an
anonymous reviewer for comments that substantially
improved the manuscript. We thank J. Brana, J. Davis, S.
DeJesus, B. Gonzales, P. Kane, M. Kimberlin, L. Lesicka,
G. Mulcahy, A. Pauli, R. Rister, T. Rutherford, M.
Santee, S. Torres, G. Verbrugge, J. Wehausen, and the late
G. Weise for assistance in the field. Funding for helicopter
surveys was provided by the California Department of
Fish and Game Mountain Sheep Conservation Program,
the Imperial County Fish and Game Commission, Desert
Wildlife Unlimited, and the United States Department of
the Interior Bureau of Reclamation. This is professional
paper 051 from the Eastern Sierra Center for Applied
Population Ecology.
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